The present invention relates generally to magnetoresistive (MR) read sensors and, more particularly, to an MR read sensor in which an insulating antiferromagnetic film provides an exchange-coupled stabilization field for the transverse bias layer in MR sensor.
A general description of the principles of operation of MR sensors in magnetic recording systems is provided by Tsang in "Magnetics of Small Magnetoresistive Sensors", Journal of Applied Physics, Vol. 55(6), Mar. 15, 1984, pp. 2226-2231. Essentially, an MR sensor detects magnetic field signals through the resistance changes of the magnetoresistive read element as a function of the amount and direction of magnetic flux being sensed by the element. MR sensors are of interest for three primary reasons: the voltage output when detecting recorded flux transitions in a magnetic medium is large and proportional to an applied sense current; good linear density resolution can be obtained; and the MR sensor output is independent of the relative velocity between sensor and medium.
It is well known in the prior art that in order for an MR sensor to operate optimally, two bias fields are required. To bias the MR material so that its response to a magnetic field is linear, a transverse bias field is generally provided. This bias field is normal to the plane of the magnetic media and parallel to the surface of the planar MR element. Typically, the transverse bias field is provided by a layer of soft magnetic material deposited adjacent to the MR element and magnetized by a magnetic field generated by a current flow in the MR element, also referred to as "soft adjacent layer" ("SAL") biasing. The transverse bias layer is separated from the MR element by a thin nonmagnetic layer.
The second bias field typically utilized with MR elements is referred to as the longitudinal bias field and extends parallel to the surface of the magnetic media and parallel to the lengthwise direction of the MR element. The primary purpose of the longitudinal bias is to suppress Barkhausen noise which is generated by multi-domain activities within the MR element. A secondary purpose of the longitudinal bias field is to improve the magnetic stability in the presence of high magnetic field excitation. The longitudinal bias field typically is provided by either hard-magnet or exchange-coupling biasing as is well-known in the art.
A prior art MR sensor as shown in FIG. 2 utilizes a magnetically soft adjacent layer as described above to provide the transverse bias field. MR sensors using a SAL for transverse bias often exhibit magnetic instability in the sensor end or tail region and significant side-track reading. For high areal storage density, greater than 1 gigabit per square inch, for example, the height of the MR stripe or element is relatively small, on the order of less than 1 micrometer (um). For element dimensions in this range, it is not possible to fully saturate the soft magnetic layer and therefore it does not provide an adequate transverse bias field to the MR sensor. For certain structural configurations, the unsaturated soft magnetic layer may also cause Barkhausen noise in the sensor. Additionally, for sensor designs using hard bias to provide the longitudinal bias field, the presence of the soft magnetic layer under the hard bias material lowers the strength of its magnetic field and thus makes it ineffective for longitudinal bias. What is needed then, is a method of stabilizing the soft magnetic layer and insuring that it is saturated.
The phenomenon of exchange anisotropy is well-known in the art. It occurs as a result of the interaction of a ferromagnetic material in contact with an antiferromagnetic material, and can be described in terms of an exchange interaction between magnetic moments on each side of the interface between the two materials. For example, exchange coupling between thin layers of ferromagnetic nickel-iron (NiFe) and antiferromagnetic iron-manganese (FeMn) produces a unidirectional anisotropy resulting in a shift of the MH loop in the MR element.
A recently developed nickel oxide (NiO) material which is antiferromagnetic and an insulator provides opportunity to solve some of the above discussed disadvantages of prior art MR sensors. Prior art of interest discussing the use of NiO for exchange coupling includes a paper entitled "Exchange Anisotropy in Coupled Films of Ni.sub.81 Fe.sub.19 with NiO and Co.sub.x Ni.sub.1-x O" by Carey et al published at page 3060 et seq. of Appl. Phys. Lett. 60, 15 Jun. 1992. Also, various papers by S. Soeya et al entitled "Magnetic Properties of NiO/NiFe Exchange Coupled Films" (1991), "A Magnetization Mechanism of Exchange-Coupled Double Layered Films" (1992), "Magnetic Properties of the Exchange Coupled Permalloy/Low Bs Ferromagnetic/Antiferromagnetic Films" (1992) and "Magnetic Properties of the Exchange Coupled NiFe/NiO Films" (1993) discuss NiO in exchange coupled systems with NiFe.